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Foreground subtraction is the removal of electromagnetic radiation (EMR) from nearer sources when more distant sources are what are of interest. This can be necessary in intensity mapping (e.g., 21-cm experiments or CMB surveys) that have the intention of studying long-term, cosmological effects, given that nearer sources are producing the same wavelength of interest through completely different means, e.g., thermal radiation from nearby sources producing some of the same wavelength of an emission line being mapped.
Sources that add such foreground EMR to more-distant observations include intervening clouds, the ISM, the IPM, the IGM, as well as Earth atmosphere. For cosmological studies, it may be that sources from within the Milky Way are unwanted. Depending upon the aim of the observation, the unwanted foreground may be of extragalactic origin such as the effects of known structures that are not the object(s)-of-study.
One means of determining the likely amount of competing radiation is through correlation of the intensity map data with data from surveys that cover sources likely to compete.